Abnormalities of respiratory rhythm such as apnea and periodic breathing occur frequently in low birth weight premature infants. Their origin has been ascribed to immaturity of the respiratory chemoreceptors. This conclusion has been supported by previous studies. A review of these studies reveals that much of the work on these infants used inappropriate stimuli, that thermal surroundings were not always well-controlled, and that other pulmonary abnormalities were not well characterized. Abnormalities of pulmonary mechanics and lung volume have been shown to determine the ability to respond to ventilatory stimuli. Work on patients with hypoxemia in early life has shown that lifelong chemoreceptor activity may be determined in infancy. These recent findings suggest that previous work on the ventilatory response and chemireceptor activity in low birthweight infants need to be repeated. The proposed studies will determine at what age chemoreceptor activity appears in low birth weight infants; if the hypoxemia experienced by these infants early in life impairs their ability to respond to hypoxic stimuli; if the apparent limitations of the ventilatory response of low birthweight infants are caused by mechanical limitations imposed by reduced lung volumes. As pointed out earlier, these studies will use stimuli appropriate to patients in this age group, the thermal environment will be carefully controlled, and other parameters of pulmonary function will be measured and correlated with changes in the ventilatory responses of low birthweight infants.